Specifications

Early "high fidelity" designs (think big E H Scott radios) had but one way to operate the output stage: Triode mode. Why? They
were the only tubes that existed at the time. For a vacuum tube configuration, triodes have the lowest plate resistance, and
therefore, make for pretty good devices to drive a speaker. But, they aren't very efficient, so they don't produce very much
power. As a result, speakers had to be large and use horn configurations to have the efficiency required to produce a usable
output. Triodes also have a limited HF response as well. But then early program material didn't have much HF energy
associated with it anyway. But then, technology greatly advanced the efficiency of a tube, resulting in the creation of the
pentode. Pentodes can produce boatloads of power compared to triodes, and have improved HF response, but have a very
high plate resistance, which ultimately makes them rather lousy for for high fidelity reproduction needs all by themselves. But
then, a solution was found.
Applying NFB to pentode based amplifiers offers the solution to the pentode's ills, because it effectively lowers the pentode's
high plate resistance characteristic, with the amount of reduction based on the amount of feedback used (assuming it was well
applied). All of a sudden then, you could have your cake, and eat it too. But of course, human beings being what we are, we
argue (to this day) over which is better: Low power triode operation (with, or without NFB), or high power pentode operation with
lots of NFB. We'll argue over the order of distortion products produced (low order for triodes, higher for pentodes), which triode
is the best triode to use, or even if a pentode connected as a triode really is a triode! Then, Misters Keroes and Hafler showed
up.
With their perfection of tapped screen operation (originally suggested by a Brit), UltraLinear Operation was born. Their
transformer company -- Acrosound -- was the first to offer it to the audio community through their first UL transformer model, the
TO-300. With it, the characteristic of a pentode tube is fundamentally changed. Now, the power, efficiency, and frequency
response of a pentode could be had, along with the low impedance and lower order distortion products of a triode, all in one
neat package. Of course, we're still human beings. So all it really did was give us a third mode of operation to argue over . But I
digress. UL changed audio history. And while good performance was still to be had in smaller amplifiers and receivers with
non-UL operation from a wave of new small power pentodes to come, UL operation (or some form of it) remained the go-to
design for most (but not all) manufactures, when it came to high power high quality audio reproduction, right up to the end of the
vacuum tube era.
All of this is likely old school to many in our hobby, but there are also many newbies coming into it everyday. If they get barraged
with decrees from those who say you must do this or that to the output stage for good sound, but have only little understanding
of basic output stage theory themselves, then they have little ability to know if a given modification is really in keeping with the
audio goals they seek. Which in fact, brings us back to the ST-70. If only for those then, this provides some background as to
why the output stage of the ST-70 was designed as it was, and, what different modification configurations can potentially do to
its performance.
The Power Output Stage Pt.3: Configuration Performance
The previous information on internal resistance and output stage configuration is important to have a basis of understanding as
to how and why various popular modifications to the output stage affect the sound as they do. The common understanding is
that it is tubes themselves that produce a unique sound quality when in reality, it is the interaction of a particular tube's
characteristic with the circuit it is installed in that produces a given sonic quality. Therefore, changing the circuit can have just as
much impact on sound quality as changing tubes can, if not more. Changing the output stage configuration in the ST-70 then is
no different.
FEEDBACK -- Claims are often made how feedback hurts performance, so some modifications result in reduced, or potentially
no feedback at all. Feedback primarily affects frequency response, distortion, and internal resistance. It would take having a
feedback modified unit on the test bench to determine the impact produced on these areas, but suffice to say, that a stock ST-
70, when operating with the NFB loop disabled, will display an internal resistance of 19 ohms on the 8 ohm tap, and a distortion
of 1.1% at 1 kHz, at 1 db below 35 watts output.
Since amplifier internal resistance can have a significant effect on the sound produced, it needs to be minimized as much as
possible, so that the speaker and amplifier can each be judged on their own merits. After all, when the characteristics of one
component affects the operation of the another, the ability to make an accurate assessment of either then is destroyed.
Besides amplifier internal resistance, the other electrical element that influences speaker damping is the DC resistance of the
voice coil itself, with a typical 8 ohm speaker often having a DC resistance of about 7 ohms or so. With a quality amplifier then, it
is the DC resistance of the speaker itself that primarily determines how effective the electrical damping action is. So what level
of internal resistance should a quality amplifier represent?
For the internal resistance of an amplifier to effectively be a non-factor, it should represent no more than 1/10th the DC
resistance of the speaker. Applying this back to the UL output stage of the ST-70 then, it can be seen that with an internal
resistance of 19 ohms on the 8 ohm tap, some help is needed from a feedback network to reduce this value low enough so as to
remove its influences on the effect of speaker operation. Even if the output stage is configured for the lowest possible internal
resistance (triode), that value (7.8 ohms on the 8 ohm tap) is still plenty high to influence the bass response of most speakers.
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